This invention relates generally to components of the hot section of gas turbine engines, and in particular, to a CVD aluminiding process for producing a modified platinum aluminide bond coat for improved high temperature performance.
In gas turbine engines, for example, aircraft engines, air is drawn into the front of the engine, compressed by a shaft-mounted rotary compressor, and mixed with fuel. The mixture is burned, and the hot exhaust gases are passed through a turbine mounted on a shaft. The flow of gas turns the turbine, which turns the shaft and drives the compressor. The hot exhaust gases flow from the back of the engine, providing thrust that propels the aircraft forward.
During operation of gas turbine engines, the temperatures of combustion gases may exceed 3,000xc2x0 F., considerably higher than the melting temperatures of the metal parts of the engine, which are in contact with these gases. The metal parts that are particularly subject to high temperatures, and thus require particular attention with respect to cooling, are the hot section components exposed to the combustion gases, such as blades and vanes used to direct the flow of the hot gases, as well as other components such as shrouds and combustors.
The hotter the exhaust gases, the more efficient is the operation of the jet engine. There is thus an incentive to raise the exhaust gas temperature. However, the maximum temperature of the exhaust gases is normally limited by the materials used to fabricate the hot section components of the turbine. In current engines, hot section components such as the turbine vanes and blades are made of cobalt-based and nickel-based superalloys, and can operate at temperatures of up to 2000xc2x0-2300xc2x0 F.
The metal temperatures can be maintained below melting levels with current cooling techniques by using a combination of improved cooling designs and thermal barrier coatings. In one approach, a thermal barrier coating system is applied to the metallic turbine component, which becomes the substrate. The thermal barrier coating system includes a ceramic thermal barrier coating that is applied to the external surface of metal parts within engines to impede the transfer of heat from hot combustion gases to the metal parts, thus insulating the component from the hot exhaust gas. This permits the exhaust gas to be hotter than would otherwise be possible with the particular material and fabrication process of the component.
Thermal barrier coatings (TBCs) are well-known ceramic coatings, for example, yttrium stabilized zirconia. Ceramic thermal barrier coatings usually do not adhere optimally directly to the superalloys used in the substrates. Therefore, an additional metallic layer called a bond coat is placed for example, by chemical vapor deposition (CVD), between the substrate and the TBC to improve adhesion of the TBC to the underlying component. In one form, the bond coat is made of a diffusion nickel aluminide or platinum aluminide, whose surface oxidizes to form a protective aluminum oxide scale in addition to improving adherence of the ceramic TBC.
The bond coat temperature is critical to the life of the TBC and has been limited to about 2100xc2x0 F. Once the bond coat exceeds this temperature, the coating system can quickly deteriorate, resulting in separation of the TBC from the bond coat. The addition of small amounts of reactive elements to platinum aluminide coatings, such as Hf, Si, Zr, and Y have been shown to increase performance of such coatings.
U.S. Pat. No. 5,658,614 to Basta et al. discloses a method of improving the oxidation resistance of a platinum modified aluminide coating applied to a nickel based superalloy substrate. In one embodiment, a hafnium-modified aluminide coating was formed using a coating gas mixture of aluminum trichloride, hydrogen and hafnium chloride. The first coating gas mixture was generated externally to a retort holding the platinum-plated workpiece that was to be coated by passing hydrogen and hydrogen chloride over a pure source of aluminum. A second mixture of Ar and HCl was generated in a separate generator external to the retort that also contained a pure hafnium bed, thereby forming hafnium chloride. The mixtures were then introduced concurrently to the coating retort forming a hafnium modified outwardly grown, single phase aluminide coating, as the gases flowed over the workpiece, depositing Hf and Al.
U.S. Pat. No. 5,989,733 to Warnes et al. is directed to a CVD outwardly grown platinum aluminide diffusion coating on a nickel or cobalt based superalloy substrate wherein the platinum modified aluminide diffusion coating is modified to include a combination of hafnium and silicon in a concentration of about 0.01% by weight (w/o) to about 8 w/o of the outer additive layer of the coating. The coating gas mixture was generated externally by passing high purity hydrogen and high purity hydrogen chloride over a high purity pure source of aluminum and then passing the mixture over a high purity pure source of silicon, both sources at 290xc2x0 C. (554xc2x0 F.) and external of the retort. A mixture of Ar and HCl was flowed in an external chloride generator through a hafnium bed at 430xc2x0 C. (806xc2x0 F.). Both gas coating mixtures were then introduced concurrently to the retort.
Prior modified platinum aluminum bond coat practice, as exemplified by the foregoing U.S. patents, involves generating two or more distinct gas coating mixtures in multiple external gas generators and concurrently introducing these mixtures into the retort.
U.S. Pat. No. 5,667,663 to Rickerby et al. discloses a method for adhering a ceramic TBC layer to a superalloy article containing a predetermined amount of aluminum. After applying a layer of platinum to the superalloy article and heat treating the article, the aluminum diffuses from the superalloy article into the platinum. In one embodiment, hafnium is added to the platinum layer by chemical vapor deposition to up to a level of 0.8 w/o. Deposition of the aluminum by CVD is not disclosed.
What is needed are improved methods to apply these reactive element modified diffusion bond coating systems. The present invention fulfills this need, and further provides related advantages.
In one form, the present invention provides both an improved method of producing coatings modified with reactive metals, such as hafnium-modified coatings and the coatings produced by that method, utilizing a single gas generation system in a diffusion coating process. An external system generates an aluminum halide gas which, as it is flowed into the system, passes over the reactive metal located between the external generator and the workpiece or parts which are located within a coating retort, and preferably the reactive metal is within, the coating retort where the parts to be coated are located. As used herein, reactive metals include Zr, Hf, V, Nb, Ta, Y, La and Ce. The reactive metal is activated by the aluminum halide gas to form a mixture of aluminum halide gases and at least one reactive metal halide gas species. In this manner, a reactive metal and aluminum are co-deposited by an internal gas generation method onto the parts to be coated as the gases pass over the parts to produce a modified single phase additive layer of platinum aluminide.
In another form, the present invention comprises a chemical vapor deposited outwardly grown platinum aluminide diffusion coating on a nickel or cobalt based superalloy substrate wherein hafnium and aluminum are co-deposited by an internal gas generation method onto the platinum-plated parts to produce a single phase additive layer of platinum aluminide with up to about 0.5 percent hafnium by weight percent, and about 1 to about 15 weight percent of hafnium in the boundary between the diffusion layer and the additive layer.
In yet another embodiment, the present invention comprises a TBC system for application to a superalloy substrate used in the hot section of a gas turbine engine. A layer of a platinum group metal is first applied to the superalloy substrate. A modified platinum aluminide diffusion coating is formed by applying, by co-deposition, aluminum and at least one reactive metal, such as hafnium, using chemical vapor deposition to the platinum group metal, the coating comprising a single phase additive layer of platinum aluminide with at least about 0.1 percent hafnium by weight (w/o) and about 0.1 to about 15 weight percent (w/o) of hafnium in the boundary formed by the diffusion layer between the substrate and the additive layer; and a ceramic thermal barrier coating applied to the modified platinum aluminide diffusion coating. The amount in weight percentage of reactive metal will vary depending upon the species selected.
While in the prior art, hafnium chloride and aluminum chloride gas coating mixtures are separately generated externally then concurrently introduced into the retort causing the hafnium and aluminum to precipitate out as they pass over the workpiece, in the present invention, there is only one external gas generator generating the gases bearing both the reactive metal species and the aluminum. In the additive layer, there is a relatively low concentration of the reactive metal so that the reactive metal is in solution with the platinum and aluminum.
One advantage of the present invention is that the coating system produced by this invention demonstrates increased service life resulting from improved spallation resistance of the TBC system.
Another advantage of the present invention is that the process consists of a single step that additionally coats the internal and external surfaces of an article to be coated with the modified platinum aluminum coating. This one step process allows for cost savings during the manufacturing process.
Still another advantage of the present invention is that the internal gas generation process provides the capability for the modification of existing CVD reactors to produce reactive element-modified coatings without the addition of new external gas generation systems.
Yet another advantage of the present invention is that the process can be easily adapted for use in airfoil material repair operations. It is advantageous to have the ability to repair a damaged section of an airfoil, rather than having to strip the entire TBC coating and completely re-bond and re-coat a TBC coating to the entire part.
Another advantage of the present invention is that the CVD coating device and process can be simplified as the device of the present invention requires only one external gas generator.
Continuing and often interrelated improvements in processes and materials, such as the improvements of the present invention, can provide cost reductions and major increases in the performance of devices such as aircraft gas turbine engines.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.